1. Technical Field of the Invention
The present invention relates generally to a laser diode, and more particularly to an improved structure of a heterojunction laser diode suitable for radar systems designed to measure the distance to a target.
2. Background of Related Art
In recent years, automotive radar systems have been proposed which measure the distance between a system-equipped vehicle and a preceding vehicle using a laser diode (i.e., a semiconductor diode) and output an alarm signal and/or apply brakes when the system-equipped vehicle is too close to the preceding vehicle. Such systems are designed to detect an object 100 m ahead, requiring laser diodes capable of being excited by pulse voltage to output a beam of light of several tens of watts (W). Specifically, unlike a low power laser diode, as shown in FIG. 26, which emits a laser beam of several milliwatts (mW), a high power laser diode, as shown in FIG. 27, has a wider light-emitting portion (i.e., a stripe width) of an active layer, which causes difficulty in producing a laser beam approximating a circular shape.
In order to alleviate the above problem, U.S. Pat. No. 5,559,819 filed on Apr. 18, 1995, assigned to the same assignee as that of this application teaches a semiconductor laser in which the sum of thicknesses of an active layer and an optical guide layer is set to 1.5 xcexcm or more for decreasing an ellipticity of a laser beam (i.e., a ratio of a major axis to a minor axis of a cross section of the laser beam). The decrease in ellipticity of the laser beam is achieved by reducing the diffraction of light in a thicknesswise direction of an optical guide layer to narrow the laser beam in that direction without changing the diffraction of light in a widthwise direction of the optical guide layer to approximate the cross sectional shape of the laser beam to a circle. Thus, there is also need for narrowing laser beam in the widthwise direction of the optical guide layer.
Specifically, a high power laser diode operates in a multimode, and the width of a light-emitting portion (i.e., a stripe width) of an active layer is great, therefore, the current flowing through the active layer has, as shown in FIG. 30, the Gaussian distribution which causes an equiphase surface of light in the laser diode to be deformed, resulting in deterioration in coherence. This will cause an output laser beam to be widened undesirably so that a far-field pattern (FFP) becomes wide, the density of an output laser beam at a remote location thereby being decreased.
It is therefore an object of the present invention to provide an improved structure of a high power laser diode capable of emitting a high density laser beam.
In in-depth analysis of internal structure of a high power laser diode to achieve the above object, the inventors of this application have found that addition of a resistor to a current path of the high power laser diode causes current distribution in an active layer (usually, Gaussian distribution) to be uniform or flat, as shown by a solid line in FIG. 31.
Specifically, a high power laser diode having a wider stripe width can be viewed, as shown in FIG. 32, as a circuit having a plurality of diodes D1, D2, . . . Dn arranged in parallel. In this circuit, it is conceivable that a large amount of current flows through each of the diodes D1, D2, . . . Dn when energized, and current values I1, I2, . . . In all exhibit the Gaussian distribution. In contrast, it has been observed that when resistors R are disposed, as shown in FIG. 33, between the diodes D1, D2, . . . Dn and the ground, it will cause the current flows to be slowed down so that they show a constant value over a wide range within a stripe width. The present invention was made based on this fact.
According to a first aspect of the present invention, there is provided a stripe-geometry heterojunction laser diode device having a stripe width of 100 xcexcm or more which comprises: (a) a laser diode chip including (1) a first electrode layer connected to a voltage source, (2) a second electrode layer connected to ground, and (3) an active layer disposed between the upper and the lower electrode layers to emit a laser beam when energized; and (b) a resistance layer having a resistance of 1 mxcexa9 or more, the resistance layer being disposed within an electric path extending from the first electrode layer to the second electrode layer through the active layer.
In the preferred mode of the invention, the resistance layer is disposed between the second electrode layer and the ground.
The resistance layer may alternatively be disposed between the first electrode layer and the voltage source.
The resistance layer is made of one of a GaAs-based material, a Si-based material, and an InP-based material.
A base material of the resistance layer may be identical with a base material of the laser diode chip.
The resistance layer has at least one side surface. The laser diode chip has at least one side surface. The resistance layer is soldered to a bottom of the laser diode chip with the side surface of the resistance layer shifted from the side surface of the laser diode chip.
According to a second aspect of the invention, there is provided a stripe-geometry heterojunction laser diode device having a stripe width of 100 xcexcm or more which comprises: (a) a laser diode chip emitting a laser beam when energized; (b) a base having disposed thereon the laser diode chip; and (c) a junction layer disposed between the laser diode chip and the base to join the laser diode chip to the base, the junction layer having a thickness which provides a resistance of 1 mxcexa9 or more.
In the preferred mode of the invention, the thickness of the junction layer is 20 xcexcm or more.
According to a third aspect of the invention, there is provided a stripe-geometry heterojunction laser diode device having a stripe width of 100 xcexcm or more which comprises: (a) a laser diode chip emitting a laser beam when energized; (b) a base having disposed thereon the laser diode chip; and (c) a junction layer disposed between the laser diode chip and the base to join the laser diode chip to the base, the junction layer having formed therein a non-junction portion so as to produce a resistance of 1 mxcexa9 or more.
In the preferred mode of the invention, the non-junction portion occupies in area 90% or more of the junction layer.
According to a fourth aspect of the invention, there is provided a stripe-geometry heterojunction laser diode device having a stripe width of 100 xcexcm or more which comprises a laser diode chip emitting a laser beam when energized. The laser diode chip includes a substrate which is made of a base material of the laser diode chip and which has a resistance of 1 xcexa9 or more.
The substrate has impurities which provides the resistance of 1 mxcexa9 or more to the substrate.
The substrate may alternatively have a thickness which provides the resistance of 1 mxcexa9 or more to the substrate.
According to a fifth aspect of the invention, there is provided a stripe-geometry heterojunction laser diode device having a stripe width of 100 xcexcm or more which comprises: (a) a laser diode chip including (1) a first electrode layer connected to a voltage source, (2) a second electrode layer connected to ground, and (3) an active layer disposed between the upper and the lower electrode layers to emit a laser beam when energized; (b) an electric path extending from the first electrode layer to the second electrode layer through the active layer; and (c) a base having disposed thereon the laser diode chip, the base having a resistance of 1 mxcexa9 or more and constituting a portion of the electric path between the second electrode layer and the ground.
In the preferred mode of the invention, the base is made of a material other than copper, silver, and gold.